Prior art electro-optical mice are disclosed in U.S. Pat. Nos. 4,364,035, 4,390,873 and 4,546,347 to Kirsch, U.S. Pat. No. 4,409,479 to Sprague et al., U.S. Pat. No. 4,521,772 to Lyon, and U.S. Pat. No. 4,543,571 to Bilbrey et al. All of these references disclose the detection of incremental movement by detecting when the intensity of light reflected from a grid comprising repetitive indicia crossed a threshold intensity level. However, none of these prior art references discloses the manner in which the value of this threshold intensity level is calibrated in relation to the range of possible intensity levels detected.
Although U.S. Pat. No. 4,390,873 to Kirsch does not disclose a method of calibrating an electro-optical mouse, the Applicant is aware that the Kirsch mouse was calibrated using a technique wherein a threshold level is selected and then tested to determine the number of samples produced during mouse travel which had a reflected light intensity greater than the predetermined threshold level and the number of samples which had a reflected light intensity less than the predetermined threshold level. This procedure would be repeated until a threshold level was selected which produced equal numbers of samples having reflected light intensities above and below the threshold level. This threshold level was then used to calibrate the mouse.